The ability of stem cells within the hematopoietic system to not only participate in normal replacement and repair but also to reprogram themselves to differentiate into unrelated cell populations combined with the knowledge that similarly pluripotent stem cells reside in multiple organ systems, has opened new fields in stem cell research. A critical question that needs to be addressed is whether these stem cells constitute a mixed population each member of which is committed to its own embryonic fate, or a pooi of truly multipotent stem cells exists within various tissues. To address this question, studies in a clinically relevant model system using single cells or clonally derived populations are required. The early gestational fetus represents a unique setting in which to assay stem cell differentiation potential and plasticity because of both the need for multiple cell types, created by the exponential growth rate of a developing fetus and the presence of a wide array of inductive environmental signals. Thus, will use the fetal sheep model that we feel is unique and ideally suited for studying the differentiative potential of highly purified human stem cells to test the underlying hypothesis that stem cell plasticity exists in various organs at a single cell level. To this end, we will address the following specific aims: I) Determine the differentiative potential/plasticity of human bone marrow derived stem cells in vivo by examining the ability of both highly purified, retrovirally marked populations of hematopoietic stem cells (HSC) and clonally derived marrow stromal cells (MSC) to not only differentiate into cells of other embryologic derivations but also to interconvert between MSC and HSC phenotypes, and 2) Test the hypothesis that stromal progenitor cells of various embryologic derivations are antigenically comparable to those found in BM and exhibit similar plasticity, by evaluating the ability of clonal population of stromal cells from endodermic, ectodermic and mesodermic derived tissues to differentiate in vivo into cells of an unrelated embryologic derivation, and performing phenotypic analysis to identify markers that are common among these cells. With these studies we hope to provide novel insights into the location of latent stem cell poois within the body and define the behavior and capabilities of these cells that are potentially of great clinical value.